The present invention relates to an electromagnetic radiation detection and storage device, to an imager incorporating it, to a process for fabricating said device and to a use of the latter. The invention applies to a device which comprises a field-effect phototransistor and is designed to form at least one imaging pixel.
Electromagnetic radiation sensors for imaging constitute an expanding field of research. In general, to produce a pixel of an imager, it is necessary to have, on the one hand, a light radiation sensor associated with a transducer for delivering an electrical signal proportional to the number of photons received over a given exposure time and, on the other hand, a resetting device intended to reset this pixel to its state prior to said light radiation.
One solution widely employed up to the end of the 1990s consisted of the technique of using matrices of charge coupled devices (CCDs), requiring a device for measuring the voltage obtained by the conversion of the charge current generated by the light radiation.
In recent years it has been endeavored to miniaturize the pixels obtained, for the purpose of associating them in active matrix form with a high pixel density and thus of providing high-resolution images.
It is within this context that image sensors based on CMOS (complementary metal-oxide semiconductor) technology have recently been developed. Each pixel of a CMOS pixel active-matrix image sensor typically contains a photodetector element (photodiode or phototransistor) and a transistor circuit intended to read the signal from the pixel). Depending on the photodetector chosen and its circuit for connection to the read transistor, the electrical signal obtained may be proportional to the dose or to the flux of photons received.
These CMOS sensors have many advantages over CCD sensors, including a lower energy consumption, easier fabrication and lower cost.
One of the main challenges in the current development of imagers is to maintain good sensitivity to the radiation while further reducing the size of the pixels used.
One known solution for significantly reducing the size of the pixels is to maximize the fill factor defined as the ratio of the sensitive area of the pixel to its total area. To do this, one known solution is to reduce the size of the “design rules” for the production of the address transistors and the transistors for amplifying and reading the signal coming from the photodetector, and of their interconnects. Another known solution is to reduce the number of transistors used for addressing and for amplifying and reading the signal generated by the photodetector. For example, such a reduction is achieved by sharing the amplification circuit between several adjacent pixels.
It is also known to use chemically functionalized carbon nanotubes or photosensitive polymer/carbon nanotube composites to obtain materials having photovoltaic properties (i.e. those generating an electric voltage or current when illuminated).
Thus, Star et al. have shown in the article “Nanotube Optoelectronic Memory Devices” by Alexander Star, Yu Lu, Keith Bradley and George Gruner, American Chemical Society, published on the Internet on 16/06/2004 that if a drop of a photoconductive polymer is deposited on a packet or “mat” of nanotubes, which is formed from an undifferentiated mixture of metal nanotubes and semiconductors and is placed between two contact electrodes and above a gate electrode, from which it is isolated by a dielectric, then a field-effect phototransistor is obtained.
More precisely, Star et al. have shown that the conductivity of this nanotube packet increases under illumination at a wavelength at which this polymer absorbs the photons and that this increase in conductivity is partly maintained after this illumination has stopped.
Star et al. have also shown in this article that the application of an AC voltage to the gate electrode of this phototransistor makes it possible to reduce the conductivity of the “mat” so as to return it to its initial value after a few minutes, the application of high gate voltages enabling the conductivity to be reset more rapidly.
A major drawback of this light detection/storage device obtained by Star et al. lies in particular in the use of this composite packet of metal nanotubes and semiconductors, which does not make it possible actually to obtain an “off” (i.e. nonconducting) state of the phototransistor and consequently a high sensitivity of the conductivity of the phototransistor to the light illumination.
Other drawbacks of this packet device of Star et al. lie in the relatively slow resetting of the phototransistor thus formed. Furthermore, the relatively large dimensions of the contact electrodes, each having a width of 1 μm and separated from one another by a distance of 50 μm, do not allow several elements to be combined for the purpose of creating a pixel matrix.